The goal of our studies is to understand the biochemical pathways and to identify key proteins (enzymes) that are involved in the activation and the effector phase of T cell-mediated cytotoxicity and exocytosis. It is expected that such knowledge will enable us to selectively manipulate different functional responses of T-cells with specifically designed immunomodulators. We recently suggested a novel mechanism of cell-to-cell interactions (an "extracellular ATP hypothesis") that may lead to cytotoxicity. According to this hypothesis high local concentrations of extracellular ATP acting alone or in concert with other molecules may induce changes in target cell plasma membrane and, eventually, result in target cell death. While testing this model we found the following: 1. CTL do indeed accumulate extracellular ATP in a Ca2+-independent manner in response to the crosslinking of TCR complex by mAb; 2. extracellular ATP can kill tumor cells, while CTL themselves are resistant to lytic effects of ATP; 3. high-levels of ecto-ATPase activity are expressed on the CTL, but not on the tumor target cell surface implicating ecto-ATP in the protection of CTL from the lytic effects of extracellular ATP; 4. extracellular ATP kills cells even in the absence of extracellular Ca++; and 5. both CTL and extracellular ATP are able to induce DNA degradation in TC; and 6. extracellular ATP is most likely acting as a "messenger" in concert with other molecules. These data indirectly support the model. However, direct evidence for such role of extracellular ATP is still required and work is in progress to develop appropriate reagents. Immunomodulation. Tle enhancement of the effector responses of CTL by inhibitors of protein phosphatase and by anti-sense RNA for the catalytic subunit of PK-A, provide the first experimental demonstration to support the possibility of being able not only to inhibit but also to enhance the CTL effector response.